cmd/compile/internal/gc: simplify typechecking definitions

There are only a handful of nodes that we need to pass to
typecheckdef (OLITERAL, ONAME, OTYPE, and ONONAME), but typecheck1
takes the awkward approach of calling typecheckdef on every node with
Sym != nil, and then excluding a long list of uninteresting Ops that
have a non-nil Sym.

Passes toolstash-check.

Change-Id: I0271d2faff0208ad57ddc1f1a540a5fbed870234
Reviewed-on: https://go-review.googlesource.com/c/142657
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/typecheck.go

@@ -296,10 +296,11 @@ func indexlit(n *Node) *Node {
 // 	n.Left = typecheck1(n.Left, top)
 func typecheck1(n *Node, top int) *Node {
 	switch n.Op {
-	case OXDOT, ODOT, ODOTPTR, ODOTMETH, ODOTINTER, ORETJMP:
-		// n.Sym is a field/method name, not a variable.
-	default:
-		if n.Sym != nil {
+	case OLITERAL, ONAME, ONONAME, OTYPE:
+		if n.Sym == nil {
+			break
+		}
+
 		if n.Op == ONAME && n.SubOp() != 0 && top&Ecall == 0 {
 			yyerror("use of builtin %v not in function call", n.Sym)
 			n.Type = nil
@@ -312,7 +313,6 @@ func typecheck1(n *Node, top int) *Node {
 			return n
 		}
 	}
-	}
 
 	ok := 0
 	switch n.Op {
@@ -3666,9 +3666,6 @@ func typecheckdef(n *Node) {
 	default:
 		Fatalf("typecheckdef %v", n.Op)
 
-	case OGOTO, OLABEL, OPACK:
-		// nothing to do here
-
 	case OLITERAL:
 		if n.Name.Param.Ntype != nil {
 			n.Name.Param.Ntype = typecheck(n.Name.Param.Ntype, Etype)